The present invention relates to aluminum-lithium alloys and more particularly to an aluminum-lithium alloy composition with good fracture toughness and high strength.
It has been estimated that current large commercial transport aircraft may be able to save from 15 to 20 gallons of fuel per year for every pound of weight that can be saved when building the aircraft. Over the projected 20 year life of an airplane, this savings amounts to 300 to 400 gallons of fuel. At current fuel costs, a significant investment to reduce the structure weight of the aircraft can be made to improve overall economic efficiency of the aircraft.
The need for improved performance in aircraft of various types can be satisfied by the use of improved engines, improved airframe design, and improved or new structural materials in the aircraft. Improvements in engines and aircraft design have pushed the limits of these technologies. However, the development of new and improved structural materials is now receiving increased attention, and is expected to yield further gains in performance.
Materials have always played an important role in dictating aircraft structural concepts. In the early part of this century, aircraft structure was composed of wood, primarily spruce, and fabric. Because shortages of spruce developed in the early part of the century, lightweight metal alloys began to be used as aircraft structural materials. At about the same time, improvements in design brought about the development of the all metal cantilevered wing. It was not until the 1930's, however, that the metal skin wing design became standard, and firmly established metals, primarily aluminum alloys, as the major airframe structural material. Since that time, aircraft structural materials have remained remarkably consistent with aluminum structural materials being used primarily in the wing, body and empennage, and with steel comprising the material for the landing gear and certain other speciality applications requiring very high strength materials.
Several new materials are currently being developed for incorporation into aircraft structure. These include new metallic materials, metal matrix composites and resin matrix composites. It is believed that improved aluminum alloys and carbon fiber composites will dominate aircraft structural materials in the coming decades. While composites will be used in increased percentages as aircraft structural materials, new low-density aluminum alloys, and especially aluminum-lithium alloys show great promise for extending the usefulness of aluminum alloys.
Herefore, aluminum-lithium alloys have been used only sparsely in aircraft structure. The relatively low use has been caused by casting difficulties associated with aluminum-lithium alloys and by their relatively low fracture toughness compared to other more conventional aluminum alloys. Aluminum-lithium alloys, however, provide a substantial lowering of the density of aluminum alloys (as well as a relatively high strength to weight ratio), which has been found to be very important in decreasing the overall weight of structural materials used in an aircraft. While substantial strides have been made in improving the aluminum-lithium processing technology, a major challenge still outstanding is an ability to obtain a good blend of fracture toughness and high strength in an aluminum-lithium alloy.